The dehydrogenation of hydrocarbons, such as paraffins, to produce olefins is an endothermic reaction and requires the input of a considerable amount of heat. For thermodynamic reasons, it has to be effected at relatively high temperatures. However at high temperatures there is also the possibility of thermal cracking. As a result the temperatures that can be employed in practice are relatively limited and so high conversions to the desired olefins are not thermodynamically possible.
In the present invention a transhydrogenation process is employed to effect dehydrogenation of hydrocarbons to olefins. In a transhydrogenation process a hydrogen-donor, such as a paraffin, is catalytically dehydrogenated in the presence of a hydrogen-acceptor such as an unsaturated compound so that the latter is hydrogenated at the same time. In effect, although this may not be the actual reaction mechanism, hydrogen is transferred from the hydrogen-donor, producing an olefin, to the hydrogen-acceptor, hydrogenating the latter.
Transhydrogenation processes for the production of olefins have been described for example in U.S. Pat. No. 3,267,170 and U.S. Pat. No. 4,684,755 wherein a hydrogen-donor such as propane, n-butane, or 2-methylpropane, has been reacted over a catalyst with a mono-olefin such as ethene as a hydrogen-acceptor. In the reaction, the hydrogen-donor is dehydrogenated to the corresponding olefin while the hydrogen-acceptor is hydrogenated to the corresponding paraffin eg ethane. It is seen that there is no net production of olefin since for each mole of olefin produced from the paraffin, one olefin molecule is consumed as the hydrogen-acceptor. Indeed there may be a net reduction in the olefin content since the aforesaid U.S. Pat. No. 3,267,170 discloses that the hydrogen-donor may be dehydrogenated further, to the corresponding diene, and/or that a mixture of the paraffin and the corresponding olefin may be dehydrogenated, by reaction with the hydrogen-acceptor olefin, to give a mixture of the olefin and diene corresponding to the paraffin. For example, it is suggested that a mixture of butane and butene-1 or butene-2, may be reacted with ethene as the hydrogen-acceptor olefin to give a mixture of butene-1, butene-2, and butadiene-1,3.
We have now discovered that that type of process may be used to give a net production of olefins by using a more highly unsaturated compound, such as a diene or acetylene, as the hydrogen-acceptor in place of the mono-olefin hydrogen-acceptors heretofore employed.
The use of more unsaturated hydrocarbons than mono-olefins as hydrogen acceptors has been described in GB-A-1046780 where it was proposed to produce cycloalkenes by the selective hydrogenation of a more highly unsaturated cycloalkene, such as cyclododeca-1:5:9-triene or cycloocta-1:5-diene, by contacting that cycloalkene with a compound having an ethylenically unsaturated hydrocarbon ring containing six carbon atoms, eg cyclohexene, as the hydrogen-donor.
As will be described hereinafter, in the present invention the hydrogen-acceptor preferably contains 3 to 5 carbon atoms, and is particularly propyne, propadiene, butadiene-1,3,2-methylbutadiene-1,3, cyclopentadiene, and/or pentadiene-1,3.
A process wherein a feedstock stream containing 2-methylpropane and n-butane, mixed with a recycle stream containing a small amount of butadiene, is reacted over a dehydrogenation catalyst to produce a mixture of butanes, butenes and butadiene is described in EP-A-166359: a butadiene-containing stream separated from the product stream is used as the aforesaid recycle stream. In this process, the amount of butadiene added to the fresh feedstock via the recycle stream is relatively small so that the amount of butadiene hydrogenation occurring is small in relation to the amount of dehydrogenation of the butanes taking place.